CO₂/CH₄ separation characteristics of poly(RTIL)-RTIL-zeolite mixed-matrix membranes evaluated under binary feeds up to 40 bar and 50°C

Three-component mixed-matrix membranes composed of a polymerized room-temperature ionic liquid (poly(RTIL)), a room-temperature ionic liquid (RTIL), and a zeolite were synthesized and evaluated for CO₂/CH₄ permeability under binary gas feeds, and operating pressures and temperatures of up to 40 bar and 50 °C, respectively. A CO₂/CH₄ selectivity of 43 with a CO₂ permeability of 202 barrers was achieved by a MMM containing 30 wt % SAPO-34 at 40 bar and 17 °C with an equimolar CO₂/CH₄ feed. The differences in selectivity and permeability between membranes containing 30 wt% SAPO-34 vs. SSZ-13 suggest that CO₂-induced swelling of the poly(RTIL) + RTIL matrix may lead to delamination of the matrix from the zeolite particle surface. This effect may detrimentally affect zeolites with a weaker surface charge. For membranes containing 30 wt % SAPO-34, at 17 °C, both CO₂ permeability and CO₂/CH₄ selectivity increase when the feed composition increases from 25 mol % to 50 mol % CO₂, likely due to competitive sorption. At 50 °C, the highest recorded selectivity was 26, which can be attributed to selectivity losses from decreased CO₂ adsorption onto the zeolites and thermally induced poly(RTIL) + RTIL matrix swelling by CO₂.